The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive using low molecular weight, low modulus polypropylene polymers. This adhesive gives high initial bond strength in spite of the long crystallization time of the polypropylene polymer. Additives are used in combination with the polypropylene polymer to increase the adhesive's crystallization time and setting speed. This is necessary for applications where high green strength or low bleed-through is required.
Hot melt adhesives are used to bond a wide variety of substrates together in a range of industrial processes. Some of these end uses include sealing cartons and corrugated boxes, labels for a wide variety of applications, and assembling disposable diapers. For some applications the hot melt is a fairly hard, flexible solid material with no surface tack immediately after adhesive application. One example would be for a carton sealing application where the product needs to “set up” or solidify quickly to hold the carton flaps in place seconds after the hot melt application. For other applications, the hot melt needs to have appreciable surface tack after it cools, for example when used as a pressure sensitive tape or label where the adhesive must bond to another substrate at room temperature.
For some other applications such as diaper construction, the adhesive is applied to the substrate in a molten state but must immediately build strength so that it will hold the article together even though there are forces acting on the adhesive bond. One common end use for hot melts is to bond the elastic strands in a diaper in place. The adhesive must be able to resist the contractive force of the elastic strands, since the elastic strands are elongated before they are bonded into the diaper. One other challenge is to ensure that the adhesive does not bleed-though the substrates it is in contact with. Nonwoven fabrics are frequently used in the production of disposable articles and care must be used so that the adhesive does not bleed-through the nonwoven. If this occurs, it can build on rollers or compressions sections of the diaper line. Many of the adhesives used to make disposable articles are pressure sensitive in nature since this tends to give the hot melt a wider process window. So a balance must be achieved between a relatively low viscosity for ease of application, fast development of internal strength to hold the substrates together immediately after being applied and resistance to bleed-through even if the adhesive is soft and/or pressure sensitive.
Typically hot melt adhesives can be based on polymers such as polyolefins (e.g. ethylene or propylene based polymers), or functionalized polyolefins (ethylene or propylene copolymers with oxygen containing monomers), or styrenic block copolymers containing at least one rubbery phase, like styrene-isoprene-styrene (SIS), or styrene-butadiene-styrene (SBS) polymers. Styrenic block copolymers are commonly used for diaper construction applications where nonwoven fabrics are frequently used. They tend to be very resistant to bleed-through on these materials. It is thought this is due to the speed at which the styrene endblocks reform after application, which happens very quickly as the hot melt cools. Hot melts that are not based on styrenic block copolymers must cool and re-crystallize after application to some degree to resist bleed-though.
Over the years, many different olefinic polymers have been used in the formulation of hot melt adhesives used in the construction of disposable soft goods. One type is amorphous polyalpha olefins, also known as APAO. They were primarily produced using Ziegler-Natta catalysis and could be made using a variety of monomers, including but not limited to propylene, ethylene and butene. Many different types of APAO copolymers and terpolymers are produced by a number of manufacturers. They include Evonik industries, who produce the Vestoplast® polymers; REXtac, LLC, who produces the Rextac RT range of materials and Eastman Chemical, manufacturers of the Eastoflex® line of polymers. They are all characterized by having a very low degree of crystallinity as measured by DSC. As commercially produced, they are random polymers having broad molecular weight distributions.
When formulated into hot melt adhesives for the construction of disposable articles, APAO's had some deficiencies due to their amorphous character. While they are useful for diaper construction applications (bonding the nonwoven to the polyethylene) they did not possess the level of elevated temperature creep resistance needed for the elastic attachment application. Another deficiency is that they tend not to spray well using conventional hot melt application equipment.
Older Ziegler-Natta catalyzed polyolefins such as polyethylene or polypropylene have not been used widely for diaper construction applications. While these polymers are used in hot melt adhesives for packaging applications (e.g. case and carton sealing), they lack the adhesion, open time and sprayability needed for disposable article construction applications. Examples of these types of polymers include the Epolene® polymers from Westlake Chemical Company, although many other manufacturers product these types of polyolefin polymers.
These older types of Ziegler-Natta catalyzed polyolefin polymers typically have very high melting points because of their high level of crystallinity. This gives a hot melt that has a very high melting point which in turn means the adhesive needs to be applied at very high temperatures, for example higher than 160° C. or even 170° C. This is undesirable since many of the substrates used in the nonwoven industry are very thin and are very sensitive to high temperatures.
More recently, polyolefins have been made using metallocene catalysis instead of the older Ziegler-Natta catalysis. Some of these new polymers have found use in the development of hot melt adhesives. However, they have not found widespread use in the manufacture of disposable articles since they tend not to spray well, their temperature application window is narrow and their adhesion to certain substrates has been poor.
The standard in the disposable industry in terms of sprayability have been hot melts based on styrenic block copolymers, particularly styrene-isoprene-styrene (SIS) block copolymers. No olefinic based polymer has been able to match the characteristics of the styrenic block copolymers in terms of ease of sprayability, performance and temperature application window.
In recent years, there has been significant changes in the way that energy has been produced, particularly in the United States. There has been tremendous increases in the amount of natural gas that is produced because of the rise of “hydraulic fracturing” or “fracking”. When natural gas is processed, there is a much higher ratio of low molecular weight constituents (C2, C3, and C4) produced versus higher molecular weight materials (C5 and higher). This translates into better availability and lower cost for olefins like polyethylene and polypropylene versus other materials where C5 through C9 monomers are required (e.g. isoprene, styrene and other aromatic monomers).
Therefore, a need exists to have a hot melt adhesive that is based on olefins, such as ethylene and propylene, to take advantage of the increased availability of natural gas and other lower molecular weight cracking feeds, but that has better performance and application characteristics than currently available polyolefin polymers.